There were many children that made up the armies of both sides but two of the youngest were eight year old Avery Brown of Ohio and eight year old Edward Black of Indiana. Both claimed to be the required 12 years old enlisting in 1861 and served as drummer boys in the Union army. Brown took a disability discharge in 1863 due to illness. Black was discharged in September 1862 but reenlisted with his father in February 1963 to become the youngest to fight in the war while still nine. He survived to the War's end and was a veteran at the age of 11.
American Civil War page